In the electronic equipment and telecommunications industries there are strenuous requirements for making electrical connection between components, often on a very small scale. Semiconductors, such as integrated circuits, are formed on wafers which are then cut into dice or chips that individually may be mounted on substrates. Typically, the substrate has fine electrically conductive circuit lines, and electrical and thermal contact must be made between the substrate and chip. As electronic appliances, such as computers, tape players, televisions, telephones, and other appliances become smaller, thinner, and more portable, the size requirements for semiconductors and the means for providing electrical connection between semiconductors and substrates, or between flexible circuits and rigid printed circuits, becomes increasingly demanding.
One method for providing electrical conductivity between two electrical elements such as between flexible circuits and printed circuits, is through the use of a Z-axis adhesive. A Z-axis adhesive (e.g. U.S. Pat. Nos. 2,822,509 and 4,606,962) typically consists of conductive particles dispersed throughout an adhesive film. When a Z-axis adhesive is used to bond together arrays of conductive elements on two insulating substrates, contact between the conductive elements is made by the conductive particles. As the demands for miniaturization continue to increase, spacing between particles and precision in spacing between particles becomes increasingly important.
A Z-axis adhesive can be made by randomly dispersing conductive metal particles in an adhesive. By randomly dispersing conductive metal particles in an adhesive, the resulting Z-axis adhesive will have random distances between individual particles, including particles touching each other. In order to provide the desired density for electrical connection of the electrical elements, sufficient particles must be loaded into the adhesive to insure that Z-axis conductivity will occur as required. It is typically necessary to load excess particles so that the maximum random gap between particles does not exceed the spacing requirements for interconnection. The random loading of conductive particles causes clumping of particles which will cause shorting of electrical elements. Therefore, it is important to be able to provide positioned conductive particles.
Accordingly, there is a need for a method for providing a patterned array of uniform metal microbeads on a substrate with spacing and precision previously unattainable. There is a need for a method that provides a regular pattern of very fine, uniform size microspheres or microbeads at precise spacing or scale previously unachievable. Such arrays can be used, for example, in a Z-axis adhesive.